1. Field of the Invention
The present invention relates to anodes for fuel cells, and to membrane-electrode assemblies and fuel cell systems including the same.
2. Description of the Related Art
Fuel cells are power generation systems that produce electrical energy through electrochemical redox reactions of oxidants and fuels (such as hydrogen or hydrocarbon-based materials such as methanol, ethanol, natural gas, or the like). Fuel cells are clean energy sources that may replace fossil fuels. A typical fuel cell includes a stack composed of unit cells, and produces varying ranges of power output. Since fuel cells have four to ten times higher energy density than small lithium batteries, they have been highlighted as small, portable power sources.
Some exemplary fuel cells include polymer electrolyte membrane fuel cells (PEMFCs) and direct oxidation fuel cells (DOFCs). DOFCs include direct methanol fuel cells which use methanol as the fuel. PEMFCs have high energy density and high power, but use hydrogen gas which must be carefully handled and requires use of accessory facilities such as fuel reforming processors for reforming methane or methanol, natural gas, or the like to produce hydrogen gas as the fuel. On the contrary, DOFCs have lower energy density than PEMFCs but the liquid fuel is easy to handle, the fuel cell may be operated at low temperatures, and there is no need for additional fuel reforming processors. Therefore, DOFCs have been acknowledged as appropriate systems for portable power sources for small and common electrical equipment.
In the above-mentioned fuel cell systems, the stack that generates electricity includes several to scores of unit cells stacked in multiple layers, and each unit cell includes a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The MEA includes an anode (also referred to as a “fuel electrode” or an “oxidation electrode”) and a cathode (also referred to as an “air electrode” or a “reduction electrode”) that are separated by a polymer electrolyte membrane. A fuel is supplied to the anode and absorbed by catalysts on the anode. The fuel is oxidized to produce protons and electrons. The electrons are transferred into the cathode via an external circuit, and the protons are transferred into the cathode through the polymer electrolyte membrane. In addition, an oxidant is supplied to the cathode, and the oxidant, protons, and electrons react on catalysts on the cathode to produce electricity along with water.